iOS8 Day-by-Day :: Day 8 :: Today Extension

Written by Sam Davies

Updated 9 Apr 2015: This post has been updated to Swift 1.2

This post is part of a daily series of posts introducing the most exciting new parts of iOS8 for developers – #iOS8DayByDay. To see the posts you’ve missed check out the introduction page, but have a read through the rest of this post first!

Introduction

Way back on day two of this blog series we took a look at the new sharing extension – which is just one of six new extension points on iOS. Today, it’s the chance of the today extension, or widget.

Widgets allow you to add content to the today screen on a device. Until iOS8, this area has been sacred – with only system apps being allowed to display anything there. This new extension point will allow you to bring small amounts of relevant info to your users, in an easily accessible fashion.

The project which accompanies this project is based around the Github user public event feed. The app itself shows the most recent events, and the today widget shows just the latest event. Throughout this post you’ll learn how to create a today extension, how to share code with the app, how to share cached data with the app and how to communicate from the widget to the app.

Creating a widget

Like all other extensions, widgets have to be distributed as part of a host app and therefore Xcode provides an extension to add a today extension target to an existing project. This sets you up with a good foundation for a widget.

At its heart, a widget is just a view controller which gets displayed within the context of the today overlay. The Xcode template includes a view controller, so you can kick off by implementing the same things that you normally would within that.

The Xcode template also includes a storyboard, already wired in, containing a simple “Hello World” label. If your layout permits, using a storyboard will allow you to create a layout which works well for a widget. It’s certainly worth using autolayout when designing your layout, as then it will cope well with different devices.

Part of the difficulty using a storyboard for this design is that in order to work well with the visual effects used on the today screen, your view should be transparent. Therefore you can be building a view without really being able to see it.

By default, a widget has a wide left margin – which will not be part of your view controller’s view. In order to alter this, a new protocol has been introduced – NCWidgetProviding. This contains methods which allow you to customise both the behaviour and the appearance of the widget. One of the methods on this protocol is widgetMarginInsetsForProposedMarginInsets() which passed you the default margin insets, and allows you to return your own version. In the GitHubToday sample project, the following override is used:

This extends the widget 30 points to the left. The design uses this space for an icon which represents the type of event:

The other method on NCWidgetProviding is called by the system to ask whether there are any updates available for the widget. This allows you to discover whether there are any updates available, update the layout if necessary, and to let the system know the result. The method is widgetPerformUpdateWithCompletionHandler(completionHandler:) and you will see a sample implementation of it from GitHubToday later in the article.

First of all, you need to address a couple of issues, the first being how to share code between the parent app and the widget.

Sharing code with the parent app

GitHubToday makes a network request to the Github API, and then parses the resulting JSON to extract the content it needs to display. This process would effectively be repeated by the widget as well as the app itself – but copying the same code between two projects is incredibly inefficient. One option would be to add the source files to both targets:

This approach will definitely work, but will result in the same functionality being created in two binaries. Luckily, there’s a better way. iOS8 introduces the concept of a dynamically library, and a widget can use the same library as the host app. Therefore the best approach is to create a dynamic framework, and put all the common code in there.

This article is not primarily about dynamic frameworks, so won’t go in to detail about how to create or use them, but once you’ve created them, you can move any shared code into it. For example, in GitHubToday, the entire model layer and networking implementation is all packaged into a dynamic framework. This includes the GitHubEvent class, along with GitHubDataProvider, which is used to make the network request itself.

Being able to share code between the app and the widget is really helpful, but widgets need to be super responsive – setting off a network operation each time the today screen comes in to view is not going to give the fast, snappy user experience you desire. In the next section you’ll learn how you can improve upon this by creating a cache which can be shared between the app and the widget.

Sharing a cache with the parent app

Since a widget is an extension it’s not allowed access to its own disc space, but it can use a shared container. If you remember back to day 2 of this series, you learnt how to create a shared container which both an extension and its host app can write to. There it was only used as a cache for a NSURLSession background task, but you can also use it as a shared cache, between the app and the widget.

You can obviously store files in this container, so you could create an SQLite database, or use CoreData, but the simplest approach here is to use NSUserDefaults as a key-value store.

Since the only data that the GitHubToday widget ever needs is the latest event, then you can store a GitHubEvent object in an NSUserDefaults file, which can live in the shared container. When the widget first opens then it can populate itself from this cached event, and then update the cache (and the view) when the network operation completes.

The following class (which exists in the common dynamic framework) demonstrates the behaviour of a simple cache:

If defines a property mostRecentEvent, which is pulled from the userDefaults if it exists. It also ensures that the event is pushed back there when it is updated. It’s important that GitHubEvent implements the NSCoding protocol, so that the keyed archiver knows how to archive and unarchive it.

The GitHubEventCache requires an NSUserDefaults object to read and write to. In order that this can be shared between the app and the extension, this must be created in the shared container using the NSUserDefaults(suiteName:) initializer:

The system will call the widgetPerformUpdateWithCompletionHandler() method once the widget had been displayed, and at this point you can kick off a network request to ensure that the latest data is displayed:

Not that here if the latest event you receive from the web service is different to the one you are currently displaying then you can update the view and then tell the system that you have new data, by calling completionHandler(.NewData). If you don’t receive new data then you can instead call completionHandler(.NoData).

Using this caching approach means that you can display the latest data when appropriate, but also maintain the responsiveness and user experience required of a today widget.

Obviously this implementation of a cache is incredibly simple – you can build as much complexity as you want in to the cache. Note that in the main app, when a network request is completed, the latest result is pushed into the cache, ensuring that the widget will always start by displaying the most recently downloaded data, whether it be from the app or the widget itself.

Navigating back to the parent app

The standard user story for a widget is that a user would look at the summary and if they want more info, then they can tap the appropriate part of the widget. In order to achieve this, then you can utilise the existing iOS URL functionality.

You can define a URL scheme in the Info section of the app’s target:

As is standard when defining a URL scheme for an app, you also need to implement the appropriate method in you app delegate:

Conclusion

All the extensions are a really cool new feature of iOS8 – they represent the beginning of Apple opening up the operating system for devs. The today extension could be hugely powerful – offering the chance to really improve the user experience. However, it’s important to use it wisely. If there’s a massive influx of mediocre widgets, then users might get annoyed enough not to trust any. Since they affect a core area of their device usage, widgets need to be good citizens.

The code for this app and widget is available on github at github.com/ShinobiControls/iOS8-day-by-day. Feel free to grab it, try it and break it. Do let me know how you get on – I’m really interested to see what widgets I’m going to be adding to my today screen in the coming months – I’m @iwantmyrealname